The color red can be associated with many powerful feelings such as anger, terror, evil, and revenge. Stalin was not a man of kindness or mercy so the red color of the letter fits his personality. To add to this effect, the "S" is on fire. Stalin had almost a burning sensation for absolute power within him. This gave him the motivation to do whatever it takes to become number one. The fire in this "S" can also signify the burning hearts of all of his people. To the average man his work as a leader what astoundin. However, he only cared about himself and did whatever it took to get what he wanted.
The firey outline and redness of the "S" signifies that Stalin's regime was not a pleasent one for the people of Russia.